1. Field of the Invention
The invention concerns improvements in a gas cooled high temperature reactor wherein cooling gas flows through the core formed by a pile of spherical fuel elements from the top to the bottom and the fuel elements attain their final state of burn-up after a single passage through the pile. More specifically, the invention relates to a high temperature reactor having a reflector surrounding the core on all sides and having absorber elements directly insertable into the core and area surrounding the core to control and to shut down the high temperature reactor. A special paired arrangement of rod shaped absorber elements and tubular absorber elements provide improvements in the reactor maintenance and control.
2. Description of the Prior Art
It is known to effect the control and/or the shutdown of the so-called pebble bed reactors by means of rod shaped absorber elments which are inserted into the bed of the fuel elements without the use of special guides such as tubes, shoulders and the like. Depending on the depth of the immersion, the absorber elements increase or decrease the flow of neutrons and thus the power generated by the reactor core.
As described in West German Offenlegungsschrift No. 12 63 939, the rod shaped absorber elements consist of an external steel sheathing, the neutron absorbing material and an internal suspension tube. To cool the rod shaped absorber elements, a partial flow of the cooling gas is conducted from the area above the fission zone through the inside of the suspension tubes and then exits through orifices in the rod tips.
From West German Offenlegungsschrift No. 23 53 653, a nuclear reactor with a bed of spherical fuel elements is known, wherein the installation for the control of power and for shutting down the reactor is designed so that the rod shaped absorber elements may be inserted through the roof reflector to a predetermined insertion depth into the space enclosed by the reflector. The installation comprises a part serving to control the reactor and another to shut it down, wherein the part serving to shut down the reactor consists of absorber rods that may be inserted into the bed and the part for the control of the reactor is formed of absorber rods displaceable within the wall of the roof reflector and the cavity enclosed by the reflector and the bed. These absorber rods, which are not inserted into the bed, are particularly effective when the nuclear reactor is operated in the known manner wherein the fuel elements pass only once through the bed. In such a case, the reactor core has an axial neutron flow distribution with a maximum in the upper third of the core.
From U.S. Pat. No. 3,365,368, a gas cooled nuclear reactor is known, wherein, above the reactor core (which does not consist of spherical fuel elements), a control and shutdown device is provided. The shutdown device comprise two groups of absorber elements, one of which consists of rod shaped elements and the other of tubular absorber elements. The rod shaped absorber elements are arranged within the tubular absorber elements and are movable therein in the fashion of a telescope. However, the absorber elements performing the normal control and shutdown functions and having a correspondingly high neutron absorption capacity in this instance are of tubular configuration, while the rod shaped absorber elements have a limited absorption capacity. Their function is to control excess reactivity. Both groups of control rods are inserted entirely into channels of the reactor core, i.e. their terminal positions are at the bottom of the channels limited by the bottom reflector. The telescopic arrangement of absorber rods in absorber tubes is provided for the purpose of being able to effect both complete shutdowns and the control of the reactivity with respect to the variation in the xenon-135 concentration with the least possible number of positions of the absorber elements.
In a nuclear reactor with a single passage of the spherical fuel elements, the side reflector is exposed to a neutron dose approximately twice as high as in the case of repeated passage of the fuel elements. Various measures to protect the side reflector have, therefore, been proposed. Thus, for example, in West German Offenlegungsschrift No. 23 47 817, a nuclear reactor with a single passage of the spherical fuel elements is described, wherein in order to protect the endangered structural elements, materials absorbing neutrons or reducing their velocity are provided within the wall of the roof reflector and the wall of the upper part of the side reflector. These materials may be present in the form of rods and housed in corresponding cavities.
In all high temperature reactors operating with a single passage of the spherical fuel elements, the rod shaped absorber elements that may be inserted directly into the bed of the fuel elements are exposed at least over their lower portion to high thermal and fast neutron fluxes of the reactor core. This is because an axial power density distribution with its maximum in the upper third of the reactor core is established in reactors of this type. As the result of the neutron irradiation, the outer steel sheathing of the absorber elements is damaged, i.e. the material becomes embrittled. Consequently, the life of the sheathing materials in the reactor core is limited. The embrittlement may be traced to two processes:
1. The effect of the dose of fast neutrons which generates lattice damage in the material, and
2. the action of the dose of thermal neutrons, which produces helium embrittlement in the metal by means of the reaction of Ni.sup.58 (n,.gamma.) Ni.sup.59 (n .alpha.)Fe.sup.56.
Estimates of the extent of embrittlement according to our present state of knowledge lead to the statement that the dose of fast neutrons permits a life of the absorber elements of eight years, but that because of the exposure to thermal neutrons this life of the absorber elements at the present time must be limited to four years.